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Q-talk *ON TOPIC ONLY* QUALITY ON TOPIC discussion of Backyard BBQ, grilling, Equipment and just outdoor cookin' in general, hints, tips, tricks & techniques, success, failures... but stay on topic. And watch for that hijacking.


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Unread 07-02-2008, 03:13 AM   #16
Hook_Line_and_Sinker
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there are alot of factors going on in a UDS that make the temperature question hard to answer

keeping in mind that thermal energy can be transported via conduction, convection, radiation or by a combination thereof, along with effect of latent heat. the only factor that can be left out is pressure, since effectively the pressure is constant, with intake and exhaust in a fixed state ( air in = air out).

latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase (i.e. solid, liquid, or gas), – also called a phase transition.

With some moisture being driven out of the cooked food and fat rendering they both absorb the heat. fat or water dripping onto the coals once again absorb heat as they are vaporized. drippings and fat can also extinguish areas in the coals which can diminish the available heat for a period of time. fat can become fuel and enrich the available heat

your temp gage only reads the "dry bulb" heat in the chamber, while all the other forms of heat energy are left unmonitored. The moist air in the chamber is more like steam than the hot air your temp gage reads.

Radiant heat travels from the heat source to the target with out heating the air in between , your gage may register some of this radiant heat due to it's stem being a small "target" . the 'heat shadow' effect that BBQchef33 mentioned can be thought of like an eclipse. Radiant heat travels like a beam of light, so anything that is hidden from the beam will NOT receive radiant heat.

convection heat can be described like holding your hand in front of the heater vent in your car, blowing heated air transfers more heat to items they are passing by, your hand gets warmer faster than the car. Again your gage may not register this heat.

Conduction heat, well in its simplest form ... think frying pan. the food in the pan will absorb the heat directly conducted from the pan. if there is a source of heat below the pan the food will eventually reach the temp of the pan. in the UDS the conductor is the smoky moist air. this is what your gage reads.

Sooooooooo 225 degrees is NOT 225 degrees. its only the average temp of the conducted heat in the air plus a small amount of the radiant heat. the rest of heat energy is totally ignored by the temp gage.

and yes the convection heat source ( the air) is 225 but it conducts more heat energy. The Radiant heat may be more or less than 225 - but odds are good that it is higher.

Practical experiment - take a non radiant heater thats has a fan and directed it to a chair, then take a radiant heater and aim it at the same chair. try the heaters on with the fan and radiant heat going - then reconfigure to no radiant no fan - you will get the idea in a hurry
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Unread 07-02-2008, 07:50 AM   #17
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Good science lesson hook. Dispite what my Spam eating friend Larry said, heat is not heat. Anyone who has cooked in a convection oven knows this. The food will cook faster due to convection. Convection is when the heat is transfered by air or liquid. Think of your forced air heating system or your car. They use convection.

Radiation is what things like the sun does. You can feel the heat the most when you are in direct exposure to it.

Conduction happens when you actually touch something. Browning food in a fry pan is a good example. Only the stuff touching the pan browns.
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Unread 07-02-2008, 08:02 AM   #18
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I use both type but using any type barer betweeen the heat and the food isnt' actually direct heat.
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Unread 07-02-2008, 08:09 AM   #19
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Quote:
Originally Posted by Hook_Line_and_Sinker View Post
there are alot of factors going on in a UDS that make the temperature question hard to answer

keeping in mind that thermal energy can be transported via conduction, convection, radiation or by a combination thereof, along with effect of latent heat. the only factor that can be left out is pressure, since effectively the pressure is constant, with intake and exhaust in a fixed state ( air in = air out).

latent heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase (i.e. solid, liquid, or gas), – also called a phase transition.

With some moisture being driven out of the cooked food and fat rendering they both absorb the heat. fat or water dripping onto the coals once again absorb heat as they are vaporized. drippings and fat can also extinguish areas in the coals which can diminish the available heat for a period of time. fat can become fuel and enrich the available heat

your temp gage only reads the "dry bulb" heat in the chamber, while all the other forms of heat energy are left unmonitored. The moist air in the chamber is more like steam than the hot air your temp gage reads.

Radiant heat travels from the heat source to the target with out heating the air in between , your gage may register some of this radiant heat due to it's stem being a small "target" . the 'heat shadow' effect that BBQchef33 mentioned can be thought of like an eclipse. Radiant heat travels like a beam of light, so anything that is hidden from the beam will NOT receive radiant heat.

convection heat can be described like holding your hand in front of the heater vent in your car, blowing heated air transfers more heat to items they are passing by, your hand gets warmer faster than the car. Again your gage may not register this heat.

Conduction heat, well in its simplest form ... think frying pan. the food in the pan will absorb the heat directly conducted from the pan. if there is a source of heat below the pan the food will eventually reach the temp of the pan. in the UDS the conductor is the smoky moist air. this is what your gage reads.

Sooooooooo 225 degrees is NOT 225 degrees. its only the average temp of the conducted heat in the air plus a small amount of the radiant heat. the rest of heat energy is totally ignored by the temp gage.

and yes the convection heat source ( the air) is 225 but it conducts more heat energy. The Radiant heat may be more or less than 225 - but odds are good that it is higher.

Practical experiment - take a non radiant heater thats has a fan and directed it to a chair, then take a radiant heater and aim it at the same chair. try the heaters on with the fan and radiant heat going - then reconfigure to no radiant no fan - you will get the idea in a hurry

um

yeah...

thats what I meant.












(Great post there HLS)
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Unread 07-02-2008, 08:15 AM   #20
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Quote:
Originally Posted by Hook_Line_and_Sinker View Post
there are alot of factors going on in a UDS that make the temperature question hard to answer

keeping in mind that thermal energy can be transported via conduction, convection, radiation or by a combination thereof, along with effect of latent heat. the only factor that can be left out is pressure, since effectively the pressure is constant, with intake and exhaust in a fixed state ( air in = air out)....
WOW! I'm impressed. No, really I am.
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Unread 07-02-2008, 08:23 AM   #21
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While I agree with the following to some extent...

Quote:
Originally Posted by CaptGrumpy View Post
I think at that low of a temp the distance from the direct source may play a role as to cooking faster than drying out a piece of meat.
Quote:
Originally Posted by OSD View Post
Distance from the fire or offsetting from the fire eases the radiant effect and then the created environment takes over. JMHO
I think we are missing one very important point. Just how much charcoal is actually burning at any point in time? On my UDS, I've gotten as much as an 18 hour burn on roughly 8 - 10 pounds of lump. To me, that's saying that there cannot be all that much burning at any point and the heat is going to be disapated rather quickly throughout the cooker, unlike a grill where you go through 4 or 5 pounds in the span of say an hour.

To tell the truth, I've been known to go through 40 pounds of lump and a dozen logs on my Lang in 12 hours. Yes, it's a larger cooker and in order to keep the temps up in the cooking chamber, I need to have a good amount of heat in the fire box. Something we just don't need in a UDS.
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Unread 07-02-2008, 09:28 AM   #22
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Direct and indirect heat are radically different in terms of cooking. In this case 250 degrees direct is not the same as 250 degrees indirect.

3 types of heat,

conduction -hot solid/liquid contact, like oil, your cooking grates or water
radiant, think the warmth you feel from standing to the side of your camp fire
convection, hot air moving around the food

The above scenario of sitting in the sun is a perfect example. The air temp can be 90 degrees in and out of the shade, however your body will take up the extra radiant heat if you are sitting in the sun. Your food is the same way.

edit,
Hook line covered all of the above
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Last edited by Jax191; 07-02-2008 at 10:04 AM..
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Unread 07-02-2008, 10:01 AM   #23
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the air in air theory if the heat is moving fast the meat will not asorbe as much heat the more heat the meat asorbe the better use of btu you are getting.
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Unread 07-02-2008, 10:19 AM   #24
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I suppose this smoker would be consider direct heat it work great but it have a baffle system betwenn fireboy and heat chamber with this much wood it will be on 275. Very good fuel /air ratio


baffle /deflector over firebox heat come out in 4 points very even heat
it think it a big green egg. I luck out on the baffle designe I suppose a blind hog find an acorn sometime

Last edited by jestridge; 07-02-2008 at 11:13 AM..
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Unread 07-02-2008, 10:46 AM   #25
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Convection
Radiant
Conduction
Fat Up
Fat Down
Slather
No Slather
Wood
Briquets
Lump
Pellets
Foil
No Foil
Blue Thermapen
Orange Thermapen
Green Thermapen
Egg
Off Set
UDS
Kettle

I used to think this was easy!
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Unread 07-02-2008, 11:33 AM   #26
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Quote:
Originally Posted by trp1fox View Post
Convection
Radiant
Conduction
Fat Up
Fat Down
Slather
No Slather
Wood
Briquets
Lump
Pellets
Foil
No Foil
Blue Thermapen
Orange Thermapen
Green Thermapen
Egg
Off Set
UDS
Kettle

I used to think this was easy!
If it was easy, everybody would do it.
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Unread 07-02-2008, 11:59 AM   #27
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Quote:
Originally Posted by trp1fox View Post
Convection
Radiant
Conduction
Fat Up
Fat Down
Slather
No Slather
Wood
Briquets
Lump
Pellets
Foil
No Foil
Blue Thermapen
Orange Thermapen
Green Thermapen
Egg
Off Set
UDS
Kettle

I used to think this was easy!
You forgot "K" orientation.
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Unread 07-02-2008, 12:39 PM   #28
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Quote:
You forgot "K" orientation.
Up or Down? Great thread guys.

Paul
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Unread 07-02-2008, 02:34 PM   #29
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up and pointed to the left... ooppss your other left...
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Unread 07-02-2008, 04:57 PM   #30
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The important part is what color the offset is. But seriously I think back in the day it was just easier to control low and slow with an offset Now we've came up with ways to control the airflow better. I don't know why the UDS wasn't built sooner.
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